Load-control backup signal generation circuit

ABSTRACT

In a case in which a malfunction occurs in a control processor which operates according to a predetermined program, a load-control backup signal generation circuit supplies a backup control signal to a switch of a load connected to an output of the control processor. The load-control backup signal generation circuit includes: a watchdog input terminal to which a watchdog signal periodically output from the control processor is input; a pulse count unit which counts a clock pulse generated with a constant period and which controls a count state of the clock pulse according to a signal input to the watchdog input terminal; and a signal selection unit which selects, from a plurality of options, a predetermined condition for causing a backup signal output unit to generate the backup control signal, based on a count output signal of a plurality of bits output from the pulse count unit.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is based on and claims priority from Japanese Patent Application (Application No. 2014-027593) filed on Feb. 17, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

One or more aspects of the present invention relate to a load-control backup signal generation circuit which supplies a backup control signal to a load switch connected to an output of a control processor, in a case in which a malfunction occurs in the control processor which operates according to a predetermined program.

2. Description of the Related Art

For example, a microprocessor for control which operates according to a predetermined program, that is, a microcomputer (control processing unit) (CPU) is embedded in various electronic control units (ECU) that are mounted in a vehicle.

The microcomputer generally performs various controls according to the content of a program that is prepared in advance, as scheduled. However, for example, in a case of being influenced by electromagnetic noise entering from the outside, a case in which a microcomputer fails, a case in which an unexpected operation is performed due to the influence of defects (bug) included in a program itself, or the like, the microcomputer can be in a runaway state.

If the microcomputer is in a runaway state, the entire electronic control device is in an uncontrollable state. Thus, a system of various electronic control devices monitors whether or not a malfunction occurs in the microcomputer, and in a case in which an occurrence of a malfunction is detected, it is necessary to return the microcomputer to a normal state.

For this reason, various electronic control devices of the related art control in such a manner that the microcomputer periodically outputs a pulse called a watchdog signal to the outside. Then, a monitoring circuit is connected to the outside of the microcomputer, and the monitoring circuit constantly monitors a watchdog signal that is output from the microcomputer. If a malfunction occurs in the microcomputer, the watchdog signal no longer appears. If the monitoring circuit detects a state where the watchdog signal does not periodically appear, an operation of the microcomputer is initialized.

In an in-vehicle electronic control device described in JP-A-2010-13988, if runaway of a main CPU 10 illustrated in FIG. 1 occurs and a pulse width of the watchdog signal WDS becomes excessive, a power supply control circuit 113 detects this and generates a reset pulse signal RST.

In addition, also in an electronic control system for vehicle described in JP-A-2011-98593, a watchdog timer 24 that is embedded in a power supply control IC 23 monitors an operational state of a CPU 1 based on a watchdog signal that is transmitted from the CPU 1, and when a malfunction occurs, transmits the reset pulse signal RST.

SUMMARY

As described in JP-A-2010-13988 and JP-A-2011-98593, a watchdog signal that is output from a microcomputer is monitored, and thereby it is possible to detect a malfunction of an operation of the microcomputer. In addition, a circuit that monitors the watchdog signal provides a reset signal for the microcomputer if a malfunction is detected. If the reset signal is provided, as is the case in turn-on of power, the microcomputer initializes a state of hardware and restarts an execution of a program from a head position.

Thus, for example, in a case in which runaway of the microcomputer is caused by temporary factors, such as input of electromagnetic noise, it is possible to return the operation of the microcomputer to a normal state by providing a reset signal.

However, in a case in which a continuous malfunction occurs in the microcomputer, it is not possible to return the operation of the microcomputer to a normal state even by providing a reset signal. In addition, in a case in which a malfunction of the microcomputer occurs in an electronic control device that controls an on-off of energizing of a load, it is not possible to perform the on-off of energizing of the load, even in a case in which a circuit that monitors a watchdog signal is mounted.

For this reason, in an in-vehicle electronic control device or the like, in preparation for a case in which a malfunction of the microcomputer occurs, it is desirable that a circuit for backup is mounted. That is, a circuit that generates a backup control signal for controlling the load instead of the microcomputer is necessary, in such a manner that the on-off of energizing of the load can be performed even in a case in which a microcomputer malfunctions.

The circuit that generates such a backup control signal can be configured using an analog circuit illustrated in, for example, FIG. 4. The analog circuit includes a time constant circuit that is configured by a capacitor Cx and a resistor Rx, and an analog comparator that compares voltages.

In the circuit in FIG. 4, if a pulse of a watchdog signal W/D that is output from the microcomputer is stopped, a voltage of the capacitor Cx increases according to a time constant with the elapse of time, as illustrated in FIG. 5, and if a predetermine time elapses, a voltage of the capacitor Cx reaches a threshold, and thus an output signal of the analog comparator is inverted. It is possible to use the output signal of the analog comparator as the backup control signal for an on-off of energizing control of the load.

However, there are following problems in the case in which the circuit illustrated in FIG. 4 is used.

(1) Since variation of the characteristics (capacitance) of the capacitor Cx is relatively great, variation of a CR time constant circuit is also increased. Thus, in a case in which an electronic control device including the circuit is mass-produced, a sufficient margin when designing is forced to be taken by taking into account the variation of a time constant due to individual differences. For this reason, it takes a time from when a malfunction occurs in the microcomputer until when a backup control signal is actually output, and the time required until when a backup control system can control the load increases.

(2) When the power is turned on or a reset signal is input, the microcomputer performs processing of initialization that is determined in advance, and starts a pulse output of the watchdog signal W/D after the processing has been completed. Thus, with regard to a circuit (for example, the circuit in FIG. 4) that monitors the pulse of the watchdog signal W/D, a time constant is determined in such a manner that even if the pulse of the watchdog signal W/D does not appear in an initialization time zone, it is not judged to be a malfunction caused by a mistake. However, with regard to the power supply that is supplied to the electronic control device or the like, there is a case in which an instantaneous interruption occurs due to various factors. Then, if the instantaneous interruption of a power supply occurs when the microcomputer performs the processing of initialization, a power supply circuit generates a reset signal, and the microcomputer restarts the processing of initialization from the beginning. In this case, the time required for the processing of initialization to be completed becomes longer than usual. According to this, the time required until the pulse generation of the watchdog signal W/D is started becomes long. Meanwhile, in the circuit of FIG. 4, when the instantaneous interruption of the power supply occurs, the charges of the capacitor Cx are discharged, but in a case in which the time of the instantaneous interruption is significantly short, the charges are insufficiently discharged. As a result, as illustrated in FIG. 6, before the microcomputer completes the processing of initialization, a voltage of the capacitor Cx reaches a threshold. Thus, despite a malfunction not actually occurring in the microcomputer, a backup control system starts an operation by mistake.

Meanwhile, it is considered that the circuit illustrated in FIG. 4 is replaced with a digital circuit. For example, by using a counter that counts clock pulses with a constant period, it is possible to detect the elapsed time from when the pulse of the watchdog signal W/D no longer appears. In addition, in a case in which the elapsed time reaches a predetermined time, it is also possible to be configured in such a manner that the backup control signal is generated.

In a case of the digital circuit described above, since a time constant circuit that performs a charge and a discharge of the capacitor Cx is not required as illustrated in the circuit of FIG. 4, it is possible to avoid an influence of the variation in the capacitance of the capacitor Cx. However, even in a case of the digital circuit, a clock pulse generator that can be inexpensively configured includes a time constant circuit that is configured with a capacitor and a resistor, and variation occurs in a period of a clock pulse by the variation (individual difference) of the capacitance of the capacitor. For this reason, by taking into account the variation of the period of the clock pulse, a sufficient margin when designing cannot be taken at a time when the backup control signal is generated.

One or more aspects of the present invention have been made in view of the above circumstances, and an object thereof is to provide a load-control backup signal generation circuit which can output a backup control signal for controlling a load in a short period of time, and can prevent a malfunction caused by an instantaneous interruption of a power supply, in a case in which a malfunction occurs in a microcomputer.

A load-control backup signal generation circuit according to one or more embodiments of the present invention described above will be briefly described in the following items (1) to (7).

(1) A load-control backup signal generation circuit which, in a case in which a malfunction occurs in a control processor which operates according to a predetermined program, supplies a backup control signal to a switch of a load connected to an output of the control processor, the load-control backup signal generation circuit including:

a watchdog input terminal to which a watchdog signal periodically output from the control processor is input;

a pulse count unit which counts a clock pulse generated with a constant period and which controls a count state of the clock pulse according to a signal input to the watchdog input terminal; and

a signal selection unit which selects, from a plurality of options, a predetermined condition for causing a backup signal output unit to generate the backup control signal, based on a count output signal of a plurality of bits output from the pulse count unit.

(2) The load-control backup signal generation circuit as described in the item (1), wherein the signal selection unit selects a count output signal of any one bit of the count output signals out of the plurality of bits output from the pulse count unit, and supplies the selected count output signal to the backup signal output unit.

(3) The load-control backup signal generation circuit as described in the item (1) or (2), wherein the signal selection unit includes: a reference value retention unit which retains a reference value corresponding to the predetermined condition; and a digital comparator which compares the count output signals of the plurality of bits that output from the pulse count unit with the reference value, and wherein an output signal of the digital comparator is supplied to the backup signal output unit.

(4) The load-control backup signal generation circuit as described in any one of the items (1) to (3), further including a selection state automatic adjustment unit which, in a state where the watchdog signal is stopped, measures an actual count state of the pulse count unit, and automatically adjusts a selection state of the signal selection unit so as to reflect the measured result.

(5) The load-control backup signal generation circuit as described in the item (1), further including:

a DC-cut capacitor connected between the watchdog input terminal and a control input terminal of the pulse count unit; and

a potential-control resistor connected between a control input terminal of the pulse count unit and a power supply line having a predetermined potential.

(6) The load-control backup signal generation circuit as described in the item (1), further including a clock pulse generator which generates the clock pulse with the constant period.

(7) The load-control backup signal generation circuit as described in the item (1), wherein the backup signal output unit retains a state of the generated backup control signal in the latch circuit, when the count output signal of the pulse count unit satisfies the predetermined condition.

According to the load-control backup signal generation circuit described in the configuration of (1), even in a case in which an instantaneous interruption occurs in a power supply, it is possible for a pulse count unit to correctly detect a length of time when a pulse does not appear in a watchdog signal. In addition, a signal selection unit appropriately selects a predetermined condition from a plurality of options, thereby being able to compensate for an influence of variation of characteristics of a component such as a capacitor, and thus it is possible to reduce a margin at the time of designing. Accordingly, it is possible to output a backup control signal in a short period of time in a case in which a control processor, that is, a microcomputer malfunctions.

According to the load-control backup signal generation circuit described in the configuration of (2), even if a huge error occurs in a period of a clock pulse due to individual differences, and an influence of the individual differences appear in a time where a count of the pulse count unit is performed, a count output signal with one appropriate bit is selected, and thereby it is possible to suppress the influence of the individual differences. Thus, it is possible to reduce a margin at the time of designing, and to output a backup control signal in a short period of time in a case in which the microcomputer malfunctions.

According to the load-control backup signal generation circuit described in the configuration of (3), even if a huge error occurs in the period of the clock pulse due to the individual differences, and the influence of the individual differences appears in the time when the count of the pulse count unit is performed, and if a reference value is determined in an appropriate value, it is possible to suppress the influence of the individual differences. Thus, it is possible to reduce the margin at the time of designing, and to output the backup control signal in a short period of time in a case in which the microcomputer malfunctions.

According to the load-control backup signal generation circuit described in the configuration of (4), since a selection state automatic adjustment unit measures actual characteristics affected by the variation due to the individual differences, and a selection state of the signal selection unit is automatically adjusted according to the measured result, it is possible to maintain an appropriate selection state, even if an adjustment operation is manually performed.

According to the load-control backup signal generation circuit described in the configuration of (5), when a pulse does not appear in the watchdog signal, it is possible to fix a control input terminal of the pulse count unit to a predetermined position, and to prevent the occurrence of the malfunction.

According to the load-control backup signal generation circuit described in the configuration of (6), since the clock pulse generator is embedded, it is not necessary to supply the clock pulse from the outside. Thus, it is possible to be used even in circumstances for an in-vehicle apparatus.

According to the load-control backup signal generation circuit described in the configuration of (7), as in a case in which a control processor malfunctions, since a level of the backup control signal is constantly maintained, it is possible to control the on-off of energizing of the load using the backup control signal.

According to a load-control backup signal generation circuit of one or more aspects of the present invention, in a case in which a malfunction occurs in a microcomputer, it is possible to output a backup control signal for controlling a load in a short period of time, and to prevent a malfunction caused by an instantaneous interruption of a power supply. In addition, it is possible to reduce an influence of variation (individual difference) of characteristics of a component such as a capacitor.

As described above, one or more aspects of the present invention are briefly described. Furthermore, by reading through a form (hereinafter, referred to as “embodiment”) for performing an invention that will be described hereinafter, with reference to the attached drawings, details of one or more embodiments of the present invention will be further clarified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electric circuit diagram illustrating a configuration example of a main unit of an electronic control device including a backup signal generation circuit according to an embodiment.

FIG. 2 is a timing chart illustrating an operation example of the backup signal generation circuit illustrated in FIG. 1.

FIG. 3 is a flowchart illustrating an operation of the backup signal generation circuit illustrated in FIG. 1.

FIG. 4 is an electric circuit diagram illustrating a configuration example of a backup signal generation circuit.

FIG. 5 is a timing chart illustrating an operation example of the backup signal generation circuit illustrated in FIG. 4, when a malfunction occurs.

FIG. 6 is a timing chart illustrating an operational example of the backup signal generation circuit illustrated in FIG. 4, when an instantaneous interruption occurs.

FIG. 7 is an electric circuit diagram illustrating a modification example of a configuration of a backup signal generation circuit according to an embodiment.

DETAILED DESCRIPTION

A specific embodiment with regard to a load-control backup signal generation circuit according to embodiments of the present invention will be described hereinafter with reference to each drawing.

<Description of Configuration Example>

FIG. 1 illustrates a configuration example of a main unit of an electronic control device including a backup signal generation circuit 20 according to the present embodiment.

An electronic control device illustrated in FIG. 1 includes a microcomputer 10 as a main control unit. The microcomputer 10 executes a program that is stored in advance in, for example, an inner memory (ROM), and thereby it is possible to perform a control for performing a function necessary for the electronic control device.

In the configuration example illustrated in FIG. 1, the microcomputer 10 has a function to control an on-off of energizing of a load 31. As a specific example of the load 31, various in-vehicle electrical components can be assumed. In addition, a switching device 32 that is configured with a power FET controls an on-off of energizing of the load 31, according to a level of a energizing control signal SG2 that is a binary signal.

The energizing control signal SG2 for controlling the load 31 is generally changed depending on a normal system control signal SG1 that is controlled by processing of the microcomputer 10. However, there is a case in which a continuous malfunction occurs in the microcomputer 10. If the microcomputer 10 malfunctions, the normal system control signal SG1 is not changed, it enters a state where the on-off control of the load 31 cannot be controlled.

The backup signal generation circuit 20 is mounted as a backup in a case in which the microcomputer 10 malfunctions (in abnormal state). That is, when the normal system control signal SG1 is not changed, a backup control signal SGB that is output from the backup signal generation circuit 20 controls the load 31 instead of the microcomputer 10.

In order to prevent a reverse flow of a signal, a diode D1 is connected between an output port 11 of the microcomputer 10 and a control input of a switching device 32, and a diode D2 is connected between a backup control signal output terminal 22 of the backup signal generation circuit 20 and the control input of the switching device 32.

During the normal operation of the microcomputer 10, a pulse of a watchdog signal W/D periodically appears in a watchdog signal output port 12 of the microcomputer 10. While the microcomputer 10 performs processing of initialization, it is not possible to output the watchdog signal W/D. In a case in which a program causes a configuration of the microcomputer 10 to runaway, the pulse of the watchdog signal W/D is stopped. Furthermore, in a case in which the microcomputer 10 malfunctions, the pulse of the watchdog signal W/D is stopped, and a potential of the watchdog signal output port 12 enters an uncertain state.

As illustrated in FIG. 1, a watchdog input terminal 21 of the backup signal generation circuit 20 is connected to the watchdog signal output port 12 of the microcomputer 10. That is, the backup signal generation circuit 20 monitors the pulse of the watchdog signal W/D, and generates the backup control signal SGB, when a state of the watchdog signal W/D satisfies a predetermined condition.

The backup signal generation circuit 20 illustrated in FIG. 1 includes a watchdog input terminal 21, a backup control signal output terminal 22, a capacitor C1, a resistor R1, a counter 23, a clock generator 24, a latch circuit 25, and a selector 26.

The clock generator 24 includes a predetermined oscillation circuit, and constantly outputs a clock pulse CK1 with a constant period. The backup signal generation circuit 20 according to the present embodiment can allow a relatively great variation (individual difference) with regard to a period of a clock pulse, and thus it may be configured with an inexpensive oscillation circuit including a CR time constant circuit with regard to the clock generator 24.

The counter 23 includes a clock pulse input terminal CK, a clear control terminal CLR, and count output terminals Q1, Q2, Q3, . . . , Qn of n bits. A clock pulse CK1 that is output from the clock generator 24 is applied to the clock pulse input terminal CK.

A clear control terminal CLR of the counter 23 is connected to the watchdog input terminal 21 via a capacitor C1, and is further grounded via a resistor R1. The capacitor C1 is provided to block DC components of an input signal. In addition, when the pulse does not appear in the watchdog signal W/D, the resistor R1 has a function that makes a potential of the clear control terminal CLR quickly approach a desired potential.

The selector 26 includes data input terminals IN1, IN2, IN3, . . . , INn of n bits, a selection control input terminal SEL, and an output terminal OUT of one bit. That is, the selector 26 selects one signal from among a plurality of signals that is applied to the data input terminals IN1, IN2, IN3, . . . , INn, according to a state of a control signal that is applied to the selection control input terminal SEL, and outputs the selected signal to the output terminal OUT.

In the example illustrated in FIG. 1, count outputs Dcnt of n bits that are output from the counter 23 are applied to the data input terminals IN1 to INn of the selector 26. In addition, the selection control input terminal SEL of the selector 26 is connected to the output port 13 of the microcomputer 10. That is, in the present embodiment, the microcomputer 10 determines a selection state of the selector 26. Furthermore, the output terminal OUT of the selector 26 is connected to the input port 14 of the microcomputer 10, and a latch circuit 25.

The latch circuit 25 is configured by a D type flip flop (D-FF). When the count outputs Dcnt of the counter 23 satisfy a predetermined condition, the latch circuit 25 generates the backup control signal SGB according to the counter output signal SGA, and has a function to retain the signal level.

A D input terminal of the latch circuit 25 is pulled up to a power supply line (VDD). In addition, in the example of FIG. 1, the counter output signal SGA that is output from the selector 26 is applied to a clock pulse input terminal CK of the latch circuit 25. The output terminal of the latch circuit 25 is connected to the backup control signal output terminal 22.

<Explanation of Operation>

FIG. 2 illustrates timing of each signal with regard to an operation of the backup signal generation circuit 20 illustrated in FIG. 1, and FIG. 3 illustrates a sequence of the operation. The operational sequence in FIG. 3 not only includes the operation of the backup signal generation circuit 20, but also includes processing which is performed by the microcomputer 10 according to a predetermined program. Operations of the backup signal generation circuit 20 and the microcomputer 10 will be described with reference to FIG. 2 and FIG. 3.

In an initial state, the backup control signal SGB is deactivated (Lo level) (S11). That is, a state of the latch circuit 25 is reset.

There is a possibility that the clock pulse CK1 that is output from the clock generator 24 is largely shifted from a value which is assumed when a period of the pulse is designed, due to an influence of variation caused by individual differences. The shifted pulse can be compensated by appropriately determining a control signal that is applied to the selection control input terminal SEL of the selector 26, in accordance with the period of an actual pulse.

Thus, in order to provide an appropriate control signal for the selection control input terminal SEL of the selector 26, before the backup signal generation circuit 20 starts a normal monitoring operation, the microcomputer 10 performs each of steps S12 to S14, S16, and S17 in FIG. 3, according to a program stored in advance.

In step S12, the microcomputer 10 outputs a predetermined control signal to the output port 13 as an initial state, in such a manner that the selector 26 selects a signal of a first bit. Furthermore, the microcomputer 10 stops an output of the pulse to the watchdog signal output port 12, and fixes the watchdog signal output port 12 to a Lo level.

If the counter 23 starts a count of the clock pulse CK1, the first bit of the count outputs Dcnt becomes Hi, and the output terminal OUT of the selector 26 goes to Hi, the processing of the microcomputer 10 proceeds to step S14 from step S13.

In step S14, the microcomputer 10 outputs a predetermined control signal to the output port 13 as a state of measurement, in such a manner that the selector 26 selects a signal of xth bit (x is arbitrary). Furthermore, the microcomputer 10 starts a measurement of time from zero second using an internal timer.

The counter 23 continues to measure the clock pulse CK1, and thus counts up according to the elapse of time (S15).

The microcomputer 10 monitors a state of the counter output signal SGA (signal of output OUT of selector 26: one bit) at the input port 14. In this case, since the selector 26 selects the xth bit, the microcomputer 10 monitors the xth bit of the count outputs Dcnt. If the xth bit of the count outputs Dcnt becomes Hi, the processing of the microcomputer 10 proceeds to step S17 from step S16.

In step S17, the microcomputer 10 stops the time measurement performed by the internal timer, and acquires the measured time value t (second) from the internal timer. Then, a pulse period T of the actual clock pulse CK1 is calculated from the time value t. T=t/(x−1)

Furthermore, the microcomputer 10 determines a control signal that is output to the output port 13, in such a manner that an actual delay from when the pulse of the watchdog signal W/D is stopped until when the counter output signal SGA (or SGB) increases, approaches an expected value. That is, based on the calculated pulse period T, an appropriate nth bit is determined, and the control signal of the output port 13 is controlled in such a manner that the selector 26 selects an nth bit of the count outputs Dcnt.

By the processing described above, since the backup signal generation circuit 20 is ready to be used, the microcomputer 10 starts a pulse output of the watchdog signal W/D to the watchdog signal output port 12 (S17).

As illustrated in FIG. 2, the clock pulse CK1 is constantly output. In addition, when the microcomputer 10 normally operates, the pulse periodically appears in the watchdog signal W/D, and if a malfunction occurs in the microcomputer 10, the pulse output of the watchdog signal W/D is suspended or stopped.

The counter 23 counts the clock pulse CK1 that is constantly input. However, if the clear control terminal CLR of the counter 23 goes to a high level (Hi) due to an occurrence of the pulse of the watchdog signal W/D, on all such occasions, the count state is cleared (S19) and returns to an initial state. Thus, when the watchdog signal W/D is normal, each of the count output terminals Q1, Q2, and Q3 of the counter 23 maintains a low level (Lo) state, as illustrated in FIG. 2.

Meanwhile, if the pulse output of the watchdog signal W/D is in a stop state, even in any case of Hi/Lo of the potential of the watchdog signal output port 12, the DC components are blocked by the capacitor C1, and furthermore the resistor R1 quickly discharges the charge stored in the capacitor C1 or the like, and thus a clear operation performed by the clear control terminal CLR is in a released state. Accordingly, the processing proceeds to step S20 from step S18 illustrated in FIG. 3.

That is, the counter 23 continues the count operation of the clock pulse CK1, and a count value is counted up according to the elapse of time. Thus, in a case in which a malfunction occurs in the microcomputer 10, as illustrated in FIG. 2, the levels of each of the count output terminals Q1, Q2, and Q3 of the counter 23 are changed.

In addition, in step S17, since the latch circuit 25 monitors the counter output signal SGA corresponding to the xth bit of the count outputs Dcnt, when the xth bit is changed from Lo to Hi (S21), the latch circuit 25 latches the Hi (S22), and switches the backup control signal SGB to Hi (S23).

Thus, in a case in which the microcomputer 10 malfunctions, the microcomputer 10 itself cannot control the energizing control signal SG2, but in this case, instead of that, the backup control signal SGB that is output from the backup signal generation circuit 20 controls the energizing control signal SG2, and thus it is possible to control an on-off of the load 31.

For example, even in a case in which an instantaneous interruption of the power supply occurs, a component (capacitor Cx in FIG. 4) with a relatively large capacitance for determining a time constant is not used, and thus it is possible to instantaneously clear the counted value of the counter 23, and to prevent a malfunction at the time of an instantaneous interruption illustrated in FIG. 6. In a method of clearing the counter 23 at the time of the instantaneous interruption, during initial processing of the microcomputer 10, the pulse may be output once to the watchdog signal output port 12, and the reset signal that is generated in a power supply circuit which is not illustrated may be provided to the counter 23.

In addition, even in a case in which variation due to individual differences occurs in the actual pulse period of the clock pulse CK1 by employing a low cost clock generator 24, it is possible to suppress the influence of the variation by appropriately adjusting the selection state of the selector 26, in the present embodiment. Accordingly, it is not necessary to expect a large margin at the time of designing, with regard to the time (delay) required from when the pulse output of the watchdog signal W/D is stopped until when the backup control signal SGB is started. For this reason, it is possible to reduce the delay, and to improve the response speed of the backup signal generation circuit 20.

In addition, in all cases, it is possible to change a condition that generates the backup control signal SGB, and to compensate for the variation of the pulse period of the clock pulse CK1, by adjusting the selection state of the selector 26.

<Explanation of Modification Example>

FIG. 7 illustrates a modification example with regard to the configuration of the backup signal generation circuit 20 illustrated in FIG. 1. A backup signal generation circuit 20B in FIG. 7 includes a digital comparator 27 and an n-bit latch circuit 28, instead of the selector 26 in FIG. 1. Other than that, the backup signal generation circuit 20B has the same configuration as that illustrated in FIG. 1.

An input terminal of the n-bit latch circuit 28 is connected to output ports 13B and 13C of the microcomputer 10. Thus, the microcomputer 10 provides a desired reference value Dref for the n-bit latch circuit 28 via the output port 13B, and the reference value Dref can be retained by a control signal of the output port 13C.

The digital comparator 27 compares count outputs Dcnt (the same as in FIG. 1) of n bits that are output from the counter 23 with the reference value Dref of n bits that are retained in the n-bit latch circuit 28, and can output a binary signal as the counter output signal SGA that indicates whether or not both of the count output Dcnt and the reference value Dref are identical to each other.

Thus, the backup signal generation circuit 20B illustrated in FIG. 7 changes the reference value Dref that are output from the microcomputer 10, and thereby it is possible to change a condition that generates the backup control signal SGB, and to compensate for the variation of the pulse period of the clock pulse CK1. In addition, the backup signal generation circuit 20B illustrated in FIG. 7 can more precisely adjust a condition that generates the backup control signal SGB than that of the configuration in FIG. 1.

<Possibility of Other Modifications>

In the configuration illustrated in FIG. 1, a case in which the microcomputer 10 automatically controls the selection state of the selector 26 is assumed, but the selection of the selector 26 may be controlled using a manually operable switch. In addition, it can be assumed that the selector 26 is controlled using data retained in, for example, a non-volatile memory. In the same manner, the configuration illustrated in FIG. 7 may also be formed in such a manner that the reference value Dref can be manually changed, and be formed in such a manner that the reference value Dref is retained in a non-volatile memory.

In addition, with regard to the count sequence (S12 to S17) illustrated in FIG. 3, various modifications are considered. That is, since a control may be performed in such a manner that a difference between an actual pulse period of the clock pulse CK1 and a pulse period that is assumed in advance is compensated, the pulse period of the clock pulse CK1 may be directly measured by the microcomputer 10, for example.

In the configuration illustrated in FIG. 1, the backup signal generation circuit 20 includes the clock generator 24, but in an environment in which an external device (not illustrated) that can output a desired clock pulse can be connected to the backup signal generation circuit 20, it is not necessary that the clock generator 24 is embedded in the backup signal generation circuit 20.

In the configuration illustrated in FIG. 1, if the pulse output of the watchdog signal W/D is stopped, and the backup control signal SGB is active, the switching device 32 (e.g., FET: Field Effect Transistor) is switched on and thereby the load 31 is in a constant energizing state. However, for example, in a case in which the load 31 is also intended to be manually controlled, it can be assumed that the energizing control signal SG2 is controlled by a logical sum or a logical product of a signal that is changed depending on a predetermined operation switch and the backup control signal SGB.

Here, the aspects of the load-control backup signal generation circuit according to the embodiments of the present invention described above will be briefly summarized and respectively described in the following items (1) to (7).

(1) A load-control backup signal generation circuit (backup signal generation circuit 20) which, in a case in which a malfunction occurs in a control processor (microcomputer 10) which operates according to a predetermined program, supplies a backup control signal (SGB) to a switch (switching device 32) of a load (31) connected to an output of the control processor, the load-control backup signal generation circuit including:

a watchdog input terminal (21) to which a watchdog signal (W/D) periodically output from the control processor is input;

a pulse count unit (counter 23) which counts a clock pulse (CK1) generated with a constant period and which controls a count state of the clock pulse according to a signal input to the watchdog input terminal; and

a signal selection unit (selector 26) which selects, from a plurality of options, a predetermined condition for causing a backup signal output unit (latch circuit 25) to generate the backup control signal, based on a count output (Dcnt) signal of a plurality of bits output from the pulse count unit.

(2) The load-control backup signal generation circuit as described in the item (1), wherein the signal selection unit selects a count output signal of any one bit of the count output signals out of the plurality of bits output from the pulse count unit, and supplies the selected count output signal (counter output signal SGA) to the backup signal output unit.

(3) The load-control backup signal generation circuit as described in the item (1) or (2), wherein the signal selection unit includes: a reference value retention unit (n-bit latch circuit 28) which retains a reference value corresponding to the predetermined condition; and a digital comparator (27) which compares the count output signals of the plurality of bits that output from the pulse count unit with the reference value, and wherein an output signal of the digital comparator is supplied to the backup signal output unit.

(4) The load-control backup signal generation circuit as described in any one of the items (1) to (3), further including a selection state automatic adjustment unit (microcomputer 10) which, in a state where the watchdog signal is stopped, measures an actual count state of the pulse count unit, and automatically adjusts a selection state of the signal selection unit so as to reflect the measured result.

(5) The load-control backup signal generation circuit as described in the item (1), further including:

a DC-cut capacitor (capacitor C1) connected between the watchdog input terminal and a control input terminal (clear control terminal CLR) of the pulse count unit; and

a potential-control resistor (resistor R1) connected between a control input terminal of the pulse count unit and a power supply line having a predetermined potential.

(6) The load-control backup signal generation circuit as described in the item (1), further including a clock pulse generator (clock generator 24) which generates the clock pulse with the constant period.

(7) The load-control backup signal generation circuit as described in the item (1), wherein the backup signal output unit retains a state of the generated backup control signal in the latch circuit (25), when the count output signal of the pulse count unit satisfies the predetermined condition. 

What is claimed is:
 1. A load-control backup signal generation circuit which, in a case in which a malfunction occurs in a control processor which operates according to a predetermined program, generates and supplies a backup control signal to a switch of a load connected to an output of the control processor, said load-control backup signal generation circuit comprising: a watchdog input terminal to which a watchdog signal periodically output from the control processor is input; a pulse count circuit which counts a clock pulse generated with a constant period and which controls a count state of the clock pulse according to a signal input to the watchdog input terminal; a signal selection circuit which selects, from a plurality of options, a predetermined condition for generating the backup control signal, based on a count output signal of a plurality of bits output from the pulse control circuit; and a latch circuit comprising a D input terminal connected to a power supply line and a latch input connected to an output of the signal selection circuit to receive an output signal of the signal selection circuit, the latch circuit outputting the backup control signal based on the power supply line in response to the output signal latching the latching circuit.
 2. The load-control backup signal generation circuit according to claim 1, wherein the signal selection circuit selects a count output signal of any one bit of the count output signals out of the plurality of bits output from the pulse count circuit, and supplies the selected count output signal to the latch circuit.
 3. The load-control backup signal generation circuit according to claim 1, wherein the signal selection circuit comprises: a reference value retention circuit which retains a reference value corresponding to the predetermined condition; and a digital comparator which compares the count output signals of the plurality of bits that output from the pulse count circuit with the reference value, and wherein an output signal of the digital comparator is supplied to the latch circuit.
 4. The load-control backup signal generation circuit according to claim 1, further comprising: a selection state automatic adjustment circuit which, in a state where the watchdog signal is stopped, measures an actual count state of the pulse count circuit, and automatically adjusts a selection state of the signal selection circuit so as to reflect the measured result. 